(1) Field of the Invention
This invention relates to a heat-minimizing spatial light modulator with image enhancement features based on diffracting digital micromirror devices (DDMDs) and to techniques for fabricating and incorporating DDMDs in projection optical imaging and patterning systems.
(2) Description of the Related Art
A maskless lithography technology that is capable of high-resolution patterning is essential for manufacturing of many microelectronic and bio-molecular devices. Lithographic techniques using conventional masks involve a multi-step process which includes defining the desired features on the mask which is then used to transfer the features onto a substrate by imaging. The process of fabricating a high-density electronic circuit involves the imaging of numerous layers (often more than 20) and requires a different mask for each layer. As the minimum feature sizes of semiconductor devices continue to shrink, the cost of masks for integrated circuit fabrication continues to increase, now exceeding a million dollars for a mask set for the leading devices.
Further, when building a prototype electronic module, the mask for each layer needs to be designed and fabricated many times in order to optimize the design of the prototype device. This leads to extremely long development times for prototyping any kind of electronic circuit. Additionally, in numerous defense applications, the number of different application-specific modules required is large, whereas the quantities needed of each type of module are not large. Since it is not possible in such a production scenario to amortize the high mask costs over large volumes, the mask costs become especially unbearable.
Thus, a maskless lithography technology that provides the required high resolution and desired patterning throughput will eliminate the need for expensive masks and remove a significant barrier to the cost-effective manufacturing of numerous electronic products. Finally, the vast majority of biotechnology applications that require rapid structuring of bio-surfaces, such as nano-texturing of bone implant surfaces, proteomics, and micro-array generation, will also be greatly enhanced if the desired patterning could be accomplished with high-resolution maskless lithography.
Spatial light modulators based on digital micromirror devices (DMDs) are used in a variety of projection optical imaging and maskless patterning applications. Such DMDs have been described with a variety of pixel elements, including tiltable, displaceable and modulatable pixel elements. Many of these DMDs can be produced in volume using semiconductor-like fabrication methods and equipment.
Special techniques of enhancing the resolution capability of DMDs, by fabricating small-area pixel mirrors, have also been described. Such techniques involve limiting the size of the reflective pixel mirror to only a small portion, such as one-eighth, of the area of the pixel element. This can result in a different problem, however, since the self-protective nature of the typical fully-mirrorized pixel element is diminished greatly—namely, heating of the spatial light modulator array by the non-reflected majority (⅞ths) of the incident radiation. Note that the typical projection light source is routinely used with suitable filters to eliminate virtually all heating by any undesired frequencies of the light source output. The desired frequencies (visible light in projection display systems, and ultra-violet radiation in patterning systems) are not usually considered dangerous sources of heat-causing energy, when used properly, i. e., without excessive absorption in any component of the system in which they are used. These desired frequencies, however, when present at power levels typical to patterning systems and projection display systems, are significant sources of heat when absorbed by the majority non-reflective areas of a spatial light modulator in which the reflective area is only a small portion of the total pixel element area. The repetitive nature of the typical patterning system, and the constant nature of the typical display system, can cause such systems to suffer damage by destructive heat buildup, e.g., degradation of the DMD, its driver electronics, optical components, and their coatings.